A childhood belief about getting older

My mother has a neurological condition where she has trouble concentrating on multiple things at once (not ADHD).
When I was a kid, I thought that this was an "adult" thing and that as I got older, I'd eventually lose my ability to concentrate on multiple things at once.
As I grew to be a teenager, I thought that I was the weird one and had some sort of mutation or other that allowed me to focus on more than one thing at a time.
It was only when I was about 16 that I learnt that having trouble focusing on multiple things at once was the exception, not the rule.